In the past, there has been known an extrusion press which uses, for example, copper, aluminum, or an alloy of the same etc. to extrude a tubular product by double-action extrusion wherein a cylinder platen and end platen are arranged facing each other, the cylinder platen is provided with a main cylinder, main ram, extrusion stem, and mandrel, the end platen is provided with a die, and a container which can advance and retract by container cylinders is provided between the extrusion stem and the die.
The extrusion stem has a dummy block arranged at its front end and is attached through a main crosshead to the main ram which is assembled in the main cylinder which is provided at the cylinder platen. At the center position of the extrusion stem, the mandrel is arranged together with the piercer cylinder rod to be able to advance and retract accompanying the extrusion stem. Further, the die is attached to the end platen facing the extrusion stem.
Between the extrusion stem and the die, the container is arranged to be able to advance and retract. A billet is contained in the container. The billet which is contained in the container is pushed by movement of the extrusion stem to the die side for completion of upsetting. After upsetting, the mandrel advances and the billet is pierced, the mandrel stops at a predetermined advancing position of the die, and the extrusion stem again advances, so the billet is extruded as a tubular product by double-action extrusion.
In a double-action extrusion press, when the front end part of the mandrel is made to stop at a predetermined position of a bearing part of the die to extrude the product, the position of the mandrel is held so that the stop position does not deviate even if the relative position of the mandrel and the bearing part of the die is pulled by the product.
PLT 1 discloses a double-action extrusion press which is provided with a piercer cylinder which is provided inside a main cylinder and a stop which is forcibly connected with the mandrel off from an axial center of the extrusion press wherein the stop acts on a hydraulic pilot valve to start the feed of a fixed amount of pressurized fluid medium to a rod-side chamber of the piercer cylinder so as to hold a predetermined axial direction position of the bearing of the die (stop position) and wherein, further, control is performed to hold the position by making the amount of feed of the pressurized fluid medium match the increase in volume of the rod-side chamber of the piercer cylinder when the mandrel is still and the main ram advances.
In this regard, in the conventional type of double-action extrusion press, the hydraulic pilot valve is switched mechanically through the stop and connecting rod and a certain amount of pressurized fluid medium is fed so as to hold the mandrel at a predetermined position of the bearing part of the die, so a delay occurs in control by exactly the amount of the movement stroke corresponding to the land of the spool of the hydraulic pilot valve. During extrusion, the front end of the mandrel therefore moves back and forth by several millimeters with respect to the predetermined stop position.
Furthermore, when changing the front end position of the mandrel or when changing the extrusion speed, it is necessary to adjust the position of the stop and to adjust the amount of fluid and pressure supplied to the rod-side chamber of the piercer cylinder each time.